1. Field of the Invention
This invention relates to an apparatus for detecting the input operation of a switch in a switching matrix provided with a plurality of switches such as a keyboard, especially relates to an apparatus for discriminating the existence of a sneak pass signal current caused by a overlapping keying operations in which a plurality of keyswitches on a key-board, for example, up to 10 key-switches, are depressed simultaneously. (condition in which a plurality of key-switches are simultaneously energized in a predetermined period.)
2. Description of the Related Art
Keyboards used as an input devices of computers and the like have recently had to have a high reliability and reduced production cost.
A sheet-like switch called a "membrane sheet" switch, is well known.
It generally includes a pair of insulating films on which are formed a plurality of fixed electrodes or movable electrodes and wiring for the same.
The films are arranged confronting each other so the fixed electrodes formed on the surface of the first insulating film are opposed by movable electrodes formed on the second insulating film with a suitable space therebetween.
This kind of sheet like switch is widely used in low cost keyboards.
Diodes are popularly used to prevent generation of a sneak pass signal current by overlapping keying, but it is difficult to mount circuit components such as diodes on a sheet-like material.
Therefore, a method for preventing generation of a sneak pass signal current in sheet-like switching devices has been required.
When diodes are used in the keyboard instead of resistors, the input detecting circuit gives a complete N key-rollover, but a practical keyboard of the N key-rollover system can be produced at a low cost using resistors, which are cheaper, are easier in handling than diodes, and can be formed in a film like configuration on a film.
A conventional keyboard in which resistor 6 and the switching elements 5 are arranged in a matrix form and are connected to a driver circuit and the detecting circuit can prevent the sneak pass signal current caused by overlapping keying of up to three keys, however there is a problem that it cannot prevent the sneak pass signal current caused by overlapping keying with more than three keys.
Not only can the conventional matrix switch utilizing resistors to detect the sneak pass signal current only discriminate the existence of the sneak pass signal current caused by overlapping keying of three keys or so, but the sneak pass signal voltage thus generated is affected by both the impedance R and the resistor 6.
Therefore, the difference between the impedance R and the value of the resistor 6 can not be made larger because of the value of the resistor 6 being limited by the impedance R.
Thus, the problem arises in that the resistor 6 must be set at predetermined values and an inexpensive method of printing resistors 6 can not be used.